A key building block of a wireless radio frequency (RF) transceiver is a low noise amplifier (LNA). A LNA amplifies an RF signal. A bias circuit typically supplies bias signals to the LNA.
FIG. 1 shows a conventional bias circuit 100 coupled to a LNA 102. LNA 102 includes a power source 104, resistors 106–108, an inductor 110, capacitors 112–114, and a bipolar transistor 116. Bias circuit 100 includes a reference current source 118, resistor 120, and a comparative amplifier 122. Bias circuit 100 provides a bias voltage 124 to LNA 102. Comparative amplifier 122 of bias circuit 100 generates bias voltage (Vbias) 124 by comparing a voltage at node 126 in LNA 102 to a reference voltage (Vref) generated by bias circuit 100 at node 128.
FIG. 2 shows an example comparative amplifier 122 in greater detail. Comparative amplifier 122 includes PMOS transistors 200–202 and bipolar transistors 204–208. Comparative amplifier 122 also typically includes a capacitor 210 coupled to node 212. Capacitor 210 generally has a large capacitance to filter high frequency supply noise and prevents the high frequency supply noise from being passed into LNA 102 (FIG. 1), where the high frequency supply noise could adversely affect performance of LNA 102 (FIG. 1).
Wireless RF transceivers are typically powered down (e.g., into a sleep mode) to conserve energy during periods of inactivity. A critical factor in performance of wireless devices is the wake up time of the device. The wake up time of a device is the total amount of time required for a device to return to a normal operating (e.g., functioning) state after having been powered down.
Referring to FIGS. 1 and 2, LNA 102 that is controlled by conventional bias circuit 100 will have a long wake up time due to the large capacitance of capacitor 210. More specifically, when a wireless transceiver is powered down, current flowing through bipolar transistor 116 is shut off (e.g., by pulling the base of bipolar transistor 116, at node 130, low). Once the current flowing through bipolar transistor 116 is shut off, the voltage at node 126 also goes high, and as a result, the voltage at node 212 shifts from a normal operating voltage to VDD (or ground depending upon the design of the comparative amplifier). During wake up, due to the large capacitance at node 212, the charging of capacitor 210 will delay for a significant amount of time the return of node 212 to a normal operating voltage.